Role of symmetry in magnetic confinement of fusion plasma
François Waelbroeck, PhD | Director del Instituto de Estudios de Fusión University of Texas.
Magnetic fusion aims to use magnetic fields in order to confine plasma heated to temperatures near 100 million degrees. A key question is the choice of magnetic configuration. The leading two concepts, the tok a mak and stellarator, both rely on nested toroidal flux sur faces ac ting as “magne tic bottles” but they differ in their symmetry proper ties. In axisymmetric configurations such as the tokamak, equi librium re qui res that the plas ma carry MegaAmperes of current. Such large plasma currents, however, give rise to dangerous disruptive instabilities that necessitate one rousavoi dance and mitigation strategies. In stellarators, by contrast, the configuration is specifically designed to minimize plasma currents but this requires breaking the symmetry.
Doing so gives rise to magnetic islands, which are ropes of flux that have a deleterious effect on confinement. Over lap of neighboring island chains causes braiding of the magnetic field and can lead to field line chaos, in which a “random walk” of the magnetic field lines allows particles and heat to escape to the wall. Magnetic islands may also appear in to ka maks as a result of spontaneous symmetry brea king. They exhibit a rich phenomenology, interacting with ions and electrons in ways analogous to the interaction of a sailboat with the wind and ocean. This talk will re view our understanding of magnetic islands and their likely role in selecting between the tokamak and the stellarator concepts for tomorrow’s fusion reactors.